The present invention relates to a switching power supply that delivers electric power from a DC power supply to a DC load via a transformer.
Referring to FIG. 19, a conventional fly-back-type switching power supply includes a bridge rectifier Rec that rectifies an AC input and produces pulsating DC power. The pulsating DC power passes through an input reactor L1 and a series diode D4 to a primary winding N1 of a transformer Tr. A switch Q1 is connected in series with primary winding N1. The series combination including input reactor L1, switch Q1 and primary winding N1 is connected in parallel with bridge rectifier Rec. A capacitor C1, preferably an electrolytic capacitor, is connected in parallel with the series combination of primary winding N1 and switch Q1. A snubber capacitor Cs is connected in parallel with switch Q1. A switch Q3 is connected between the output of input reactor L1 and a common connection of bridge rectifier Rec.
When switch Q1 is ON, energy is stored in primary winding N1. When switch Q1 is OFF, stored energy is released through a secondary winding N2. The output voltage is regulated by controlling the ON and OFF times of switch Q1.
In the circuit of FIG. 19, so-called soft switching (zero voltage switching), causes switch Q1 to switch ON when the voltage across snubber capacitor Cs is at its lowest value. This is accomplished by selecting values of the leakage inductance of primary winding N1 and the capacitance of snubber capacitor Cs so that these elements resonate at the switching speed. Soft switching reduces power loss and improves noise suppression.
Switch Q3 is switched ON to produce input current flow through input reactor L1. The input current flow improves the power-factor of the circuit. When switch Q3 is switched ON, energy stored in the input reactor L1 is fed to electrolytic-type capacitor C1. Switching switch Q3 ON and OFF improves the power-factor even when the input voltage is low, since input current flows whenever the switching power supply is operating.
The OFF-period of switch Q1 is set to a length of time determined by the resonant frequency of the series combination of the leakage inductance of primary winding N1 and snubber capacitor Cs. The OFF-period of switch Q1 must be related to the resonant frequency to produce soft switching in the switching power supply of FIG. 19. In contrast, the output voltage is regulated only by the ON-period of switch Q1. Since the ON-period and the OFF-period of switch Q1 are governed by different criteria, the switching frequency of switch Q1 must therefore vary to regulate the output voltage while maintaining soft switching.
Switching power supplies used in television sets and display devices have switching frequencies that are generally synchronized with the deflection frequency. Therefore, a conventional switching power supply that depends on its switching frequency to regulate output voltage is not useful in such variable frequency applications.
The use of two separate switches Q1, Q3 for voltage regulation and power-factor improvement respectively, increases the noise level of the resulting output of the switching power supply. In addition, diode D4 in series with switch Q1 causes a voltage drop when current flows and decreases the switching power supply efficiency.